"immutable field" errors are confusing when the handle is mutable, should describe why it is immutable

[huonw]

struct Foo { a: usize }
fn main() {
    let mut x = std::rc::Rc::new(Foo { a: 1 });
    x.a += 1;

    let mut y = &(Foo { a: 1 });
    y.a += 1;
}

<anon>:4:5: 4:13 error: cannot assign to immutable field
<anon>:4     x.a += 1;
             ^~~~~~~~
<anon>:7:5: 7:13 error: cannot assign to immutable field `y.a`
<anon>:7     y.a += 1;
             ^~~~~~~~

Gives no indication that the field is immutable because it is stored inside an Rc/&. These example are somewhat obvious, since the initialiser is directly above, but it's certainly possible to be matching deep inside some data structure and meet this error even though there are muts on all the appropriate variables.

The error message could include: "note: this field is immutable because it is being accessed via an & reference [created by the autoderef of a Rc]".

[steveklabnik]

Triage: same problem, but some updated code:

struct Foo { a: usize }
fn main() {
    let mut x = std::rc::Rc::new(Foo { a: 1 });
    x.a += 1;

    let mut y = &(Foo { a: 1 });
    y.a += 1;
}

[steveklabnik]

Triage: no change

[estebank]

Related: #47774.

[estebank]

Current output:

error[E0594]: cannot assign to field of immutable binding
 --> src/main.rs:4:5
  |
4 |     x.a += 1;
  |     ^^^^^^^^ cannot mutably borrow field of immutable binding

error[E0594]: cannot assign to field `y.a` of immutable binding
 --> src/main.rs:7:5
  |
7 |     y.a += 1;
  |     ^^^^^^^^ cannot mutably borrow field of immutable binding

[estebank]

Current output:

error[E0594]: cannot assign to data in a `&` reference
 --> src/main.rs:4:5
  |
4 |     x.a += 1;
  |     ^^^^^^^^ cannot assign

error[E0594]: cannot assign to `y.a` which is behind a `&` reference
 --> src/main.rs:7:5
  |
6 |     let mut y = &(Foo { a: 1 });
  |                 --------------- help: consider changing this to be a mutable reference: `&mut (Foo { a: 1 })`
7 |     y.a += 1;
  |     ^^^^^^^^ `y` is a `&` reference, so the data it refers to cannot be written